Basically, melt-coating has been effected hitherto by pressing a body of the coating material against the substrate while one is moving relative to the other, and at the same time causing material to melt from the face of the body that rubs against the substrate.
In some instances, the heat for melting the coating material has been generated as a result of frictional heating arising from the pressure and movement between the body and the substrate. In these instances the pressure between the body and substrate is necessarily high. Reference may be made to the following patent specifications, which exemplify this known melt-coating technique: U.S. Pat. No. 4,959,241 (Thomas et al), U.S. Pat. No. 4,930,675 (Bedford et al) and U.S. Pat. No. 3,553,007 (Hennig).
In other instances, the substrate has been preheated so that the surface of the body rubbing against the substrate is heated primarily by conduction from the substrate. In these instances the pressure between the body and the substrate may be lower, but it has always been at least sufficient to ensure the production of a continuous deposit on the substrate. Typical examples of this melt-coating technique are disclosed in patent specifications U.S. Pat. No. 3,630,802 (Dettling), U.S. Pat. No. 3,551,184 (Dremann et al), U.S. Pat. No. 2,327,739 (Peters) and applicants' own AU 10071/92.
The present invention is concerned with the last mentioned technique wherein the heat is supplied by pre-heating the substrate. In relation to that technique, the rubbing pressure has to be carefully controlled or some additional procedure has to be taken to obtain a finished coating of the required thickness.
Thus, according to U.S. Pat. No. 3,551,184 an adherent, thin film of metallic lithium is first formed by "wetting" the substrate with lithium while the substrate is well above the melting point of lithium, and that adherent film is then made thicker by depositing molten lithium on it; similarly, in U.S. Pat. No. 2,327,739 an initial thin film of selenium, formed by careful control of the substrate temperature to a value just above the melting point of selenium, is itself overlaid with one or more further deposits to reach a desired thickness; in U.S. Pat. No. 3,630,802 a metering gate controls the thickness of an excessively thick deposit of polymeric material on a carrier strip, presumably with undesirable spillage from the strip edges; whereas in AU 10071/92, other relevant variable parameters, such as substrate speed and temperature, are kept constant, and the pressure between a block of polymeric paint composition and a steel strip is adjusted to establish a deposition rate corresponding to the required thickness of the finished coating, and then kept constant so as to maintain that rate.
Thus, in the methods of AU 10071/92, the thickness of the finished coating may be seen as a constant parameter of a steady state condition established by maintaining all the relevant factors, including the load on the body determining the pressure between the body and the substrate, substantially constant. However, in practice it is not feasible to keep such factors as substrate temperature and body consistency perfectly constant, so that some undesirable variation in coating thickness invariably occurs.